Abstract Bipolar disorder (BD) is a frequently devastating psychiatric illness that is challenging to diagnose and treat. Identifying the underlying mechanisms of this illness may provide a foundation for better evidence-based diagnostic and therapeutic techniques. For the first time in the context of psychiatric illness, we recently explored the utility of a magnetic resonance imaging (MRI) strategy called T1 relaxation in the rotating frame (T1?), which is highly sensitive to brain pH. We tested patients with BD in the euthymic state and found prominent T1? differences compared to matched controls. More recently, we also imaged participants with BD in depressed and manic states. These studies suggest that the brain area with the most prominent T1? changes in bipolar disorder is the cerebellum, a structure has been previously suggested to contribute to BD but has received relatively little attention compared to forebrain structures. Neuroanatomical models of BD have largely overlooked the cerebellum despite compelling evidence that the cerebellum is strongly connected to brain regions involved in the emotional control network that has been put forth as a model of the disorder. We hypothesize that cerebellar activity plays a critical role in regulating mood in BD, which will be tested in this proposal using a cross-sectional design and recruiting BD subjects across the mood spectrum as well as matched controls. Participants will undergo psychiatric symptom assessment and brain imaging. Psychiatric symptom assessments will include, the Montgomery-Asberg Depression Rating Scale (MADRS) and Young Mania Rating Scale (YMRS). Brain imaging will include quantitative whole-brain T1? mapping, 31P- and 1H- MRS of the cerebellar vermis, as well as diffusion imaging (DWI) and resting state fMRI. Medications will be assessed and used as covariates in analyses. This data will be used to assess the following aims: Aim 1) Does cerebellar activity play a significant role in mood regulation in BD? We hypothesize that the cerebellum plays a significant role in maintaining a euthymic mood state (i.e., plays a compensatory role). We reason that if cerebellar activity normalizes mood, then its activity should be greatest when BD participants are euthymic and decrease with increasing mood symptom severity. Alternatively, if cerebellar activity drives abnormal moods, then it is likely to be greatest in patients who are manic or depressed. Aim 2) Does connectivity of the cerebellar vermis with the emotional control network differ with mood symptoms in BD? We hypothesize that functional connectivity between the vermis and nodes of the emotional control network will vary with mood state with increased connectivity in the euthymic state and decreased connectivity during exaggerated mood states (depression/mania). We also expect that BD participants in the euthymic state will exhibit increased connectivity relative to healthy controls. These results would provide further evidence that the cerebellum is playing a compensatory role to maintain mood. In addition, this would support a refined model of the neural circuits underlying the pathophysiology of BD.